Obtaining glycols of low aldehyde content

ABSTRACT

A process for obtaining glycols of low aldehyde content in which the plant used to obtain the glycol(s) is surface-treated, in whole or in part, with at least one reductive phosphorus compound.

This application is a 371 of PCT/EP97/00271 filed Jan. 21, 1997.

The present invention relates to a process for obtaining glycols of lowaldehyde content and to products prepared using these glycols.

Glycols of low molecular mass, such as mono-, di- and triethyleneglycol, are important products of the chemical industry. Monoethyleneglycol in particular (also referred to as 1,2-ethanediol, ethyleneglycol or simply MEG) is among the principal products of the chemicalindustry worldwide, and is used predominantly as antifreeze for vehicleradiators and as raw material for the production of polyesters.

The only process currently used for the large-scale industrialproduction of ethylene glycol comprises the hydrolysis of ethylene oxideand the subsequent working up of the resulting reaction mixture.Worldwide production capacity for ethylene glycol produced by ethyleneoxide hydrolysis is currently estimated at 7×10⁶ metric tons per annum.In this preparation process, the ethylene oxide is reacted continuouslyor in batches with water, in special reactors and under appropriateconditions. The resulting aqueous reaction mixture is then concentratedover several stages, and the crude glycol, finally, is purified byfractionation (cf. eg.: K. Weissermel, H.-J. Arpe, IndustrielleOrganische Chemie [Industrial Organic Chemistry], 3rd ed., VCH 1988, p.159 ff.). The principal components of the reaction mixture are typicallymono-, di- and triethylene glycol. Tetraethylene glycol and higherhomologs are usually present in quantities so small that they aregenerally not worth recovering.

Distillation processes and apparatus for the purification of glycols, ofvarious design, are known (cf. eg.: Ullmanns Encyklopadie dertechnischen Chemie [Ullmann's Encyclopedia of Industrial Chemistry], 4thedition, VCH 1974, Volume 8, p. 200 ff.). In the majority of cases, inserial distillation columns, first water and then ethylene glycol and,finally, the higher glycol ethers are recovered continuously. Variousunits can be used as evaporators for the distillations; moderncontinuous plants, however, employ falling-film evaporators for energyreasons. For reasons of cost, these plant components are commonly madefrom carbon steel.

Owing to the high boiling points of ethylene glycol and its homologs,the distillations always take place under reduced pressure. Any vacuumdistillation unit, whether industrial-scale or laboratory-scale,possesses a certain leakage rate, ie. leaks which to a minor extentallow the surrounding atmosphere to enter the unit. Thus, in the courseof operation, atmospheric oxygen finds its way into the vacuumdistillation.

Like all alcohols, glycols can readily be oxidized both thermally(autoxidation) and catalytically. The reaction products of ethyleneglycol with oxygen or other oxidizing agents are aldehydes (glycolaldehyde, glyoxal, formaldehyde, acetaldehyde) and the correspondingacids. On the other hand, however, especially when ethylene glycol isused to produce polyester films, there are particularly high purityrequirements. Films produced using aldehyde-rich ethylene glycol proveto be highly sensitive to light. In particular, slight yellowing can beobserved. Therefore, in the course of polyester production but also inother sectors, the presence of these oxidation products is extremelyundesirable, so that the users are attempting to limit the aldehydecontent by means of strict specification. For example, the aldehydecontent of ethylene glycol intended for film production should be lessthan 20 ppm.

Provided no particular effects occur in the course of ethylene glycolproduction, the quantity of oxidation products formed is normallyunimportant. However, it has been observed in industrial plants that arise in the proportion of aldehyde in the distilled glycol may occurwhich was not readily explainable. At the same time, unusually largequantities of rust particles (magnetite) were found in the liquid phasesof the distillation.

The problem set out above of increased aldehyde formation should alsoexist in the context of the distillative purification of ethylene glycolwhich has been prepared by other synthesis routes, for example thecatalytic oxidation of ethylene in acetic acid.

Moreover, it is known that in large-scale industrial processes such as,for example, the manufacture of polyester fiber, large quantities ofliquid, glycol-containing residues are produced. Recovery of the glycolspresent in these residues is reasonable from an economic standpoint.Finally, large quantities of ethylene glycol are produced in the form ofused radiator fluid or used antifreeze, from which the glycol canlikewise be recovered. In the distillative working up of these wastes,however, there is likewise the problem of unwanted aldehyde formation. Afurther factor is that the wastes may already contain small quantitiesof aldehyde, presenting an additional hindrance to the preparation ofpure, ie. essentially aldehyde-free, glycols.

The term "glycol" as used herein embraces monoethylene glycol inparticular and also the distillable homologs thereof, such as di-, tri-and tetraethylene glycol.

It is an object of the present invention, therefore, to provide aprocess which makes it possible to obtain glycols whose aldehyde contentis markedly reduced. A particular object is to provide monoethyleneglycol which reliably conforms to the strict specifications applying tostarting materials for the production of polyester.

Surprisingly, we have found that this object is achieved by providing aprocess for obtaining glycols of low aldehyde content fromglycol-containing mixtures, in which the plant used for the recovery ofglycol is surface-treated, in whole or in part, with at least onereductive phosphorus compound. The intention is to treat at least thoseparts of the plant which come into contact with a glycol in the courseof working up the mixture.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows a process scheme and apparatus components by whichethylene glycol is produced by an embodiment of the invention.

The surface treatment is intended in particular to be carried out on theinner faces of those parts of the plant which are permanently ortemporarily in contact with a glycol in vapor form during the working upof the mixture. This applies in particular to plant components whereprocesses are carried out at elevated temperature and to those producedfrom corrodable material, for example steel. This may be the case, forexample, with evaporators and their feed and discharge pipes,distillation columns and reactors. In all of these plant components itis possible for vapor spaces to form in which the unwanted formation ofaldehyde can take place at the temperatures prevailing therein.

In accordance with the invention this surface treatment can be carriedout discontinuously or continuously. In the case of a discontinuousprocedure, the plant components required for separating the mixture canbe treated, in whole or in part, with the phosphorus compound before themixture is processed, for example, the distillation column used for thevacuum distillation of monoethylene glycol can be treated with thephosphorus compound if monoethylene glycol low in aldehyde is to beproduced. For the discontinuous procedure, the surface treatment can becarried out, for example, before processing each batch of mixture to beworked up, or else at other appropriate intervals of time. This isdependent on the severity of the reformation of aldehyde which isobserved in the case of separation.

If, on the other hand, operation is continuous, then the phosphoruscompound can be added to the glycol-containing mixture prior toseparation. The phosphorus-containing mixture can then be separated inthe plant. In accordance with the invention it is particularly preferredto meter the phosphorus compound into the mixture continuously, directlybefore separation. This is particularly advisable if it is expected thatthe mixture to be separated contains aldehyde even before separation. Bythis means it is possible to reduce these aldehydes by means of thephosphorus compound even before the actual separation step. If it isdesired, for example, to prepare monoethylene glycol low in aldehyde,then the phosphorus compound can be added continuously, directly beforeintroduction into the vacuum distillation column which is used formonoethylene glycol separation, to a glycol-containing mixture produced,for example, by continuous hydrolysis of ethylene oxide.

A combination of continuous and discontinuous procedure is likewiseconceivable.

The phosphorus compound used in accordance with the invention may be anyorganic or inorganic phosphorus compound or a combination of organic andinorganic phosphorus compounds showing the desired reductive effect. Thephosphorus compound applied comprises a reductive p⁺³ -containingspecies. Preferably it is chosen from phosphorous acid (which ispredominantly in the stable tautomeric form HP(O)(OH)₂, which is alsocalled phosphonic acid; cf. Rompp Chemie Lexikon [Rompp's ChemicalDictionary], 9th ed.) and the salts thereof. Salts of phosphorous acidare preferably selected from water-soluble salts such as, in particular,alkali metal phosphites, zinc phosphites and calcium phosphites.Particularly preferred alkali metal phosphites are sodium phosphite andpotassium phosphite. The corresponding hydrogen phosphites can also beused. What is most preferred, however, is to use the acid itself.

If the phosphorus compound is added to the mixture itself, this additionis usually made such that the phosphorus compound is present in aproportion of from about 10 to about 5000 ppm before glycol is removedfrom the mixture. The proportion should preferably be in the range fromabout 100 to about 1000 ppm, and should in particular be about 500 ppm.The phosphorus compound is preferably added as a solution in a glycol,such as monoethylene glycol.

If the plant is to be surface-treated discontinuously with thephosphorus compound, then a solution is prepared containing thephosphorus compound in a proportion of about 0.1 to 10% by weight,preferably from about 0.5 to 5% by weight, based on the overall weightof the solution. The phosphorus compound can be dissolved in a suitableorganic solvent, for example monoethylene glycol, or in an aqueoussolvent, for example water. An aqueous solution is preferably used. Thesolution prepared in this way is then used to treat those parts of theplant which are required for separating the mixture. Treatment can becarried out, for example, by flooding the relevant parts of the plantwith the solution and incubating them for a suitable period. Analternative possibility would be flushing of the parts of the plant,allowing the solution to circulate for an appropriate period by pumpingit around the plant parts. Also conceivable is spraying or irrigation ofthe parts of the plant. The necessary treatment period can be determinedby the skilled worker without great difficulty, and may, for example, befrom about 2 to 8 hours, for instance 4 to 6 hours. Treatment can becarried out at ambient temperatures or, for example, with heated aqueoussolution (heated for example at 40-80° C.).

Working up of the glycol-containing mixture in accordance with theinvention produces, in particular, ethylene glycol of surprisingly lowaldehyde content, the latter being in the region of less than about 20ppm, for example about 10-20 ppm. Surprisingly, the novel process makesit possible to conform reliably to the strict specifications forethylene glycol as apply, in particular, to the production of polyesterfibers and films.

Without being limited to the following explanation, the surprisingeffect observed in accordance with the invention can be explained asfollows:

The autoxidation of ethylene glycol, ie. the direct reaction with oxygenwithout a catalyst, is a slow process. In laboratory experiments whereglycol is subjected, under the conditions of a typical vacuumdistillation, to the oxygen which penetrates as a result of the naturalleakage rate, only a slow and small increase in the aldehyde levels isobserved. This is particularly the case when these experiments arecarried out in apparatus made of inert materials (e.g. glass). Evenartificial leakage rates involving increased supply of oxygen lead to nodrastic worsening of these levels.

If, however, iron bodies (eg. filings) are introduced into the sameexperimental setup in such a way that they are able to come into contactwith the vapor phase over the distilled glycol, then, at an identicalleakage rate, aldehyde contents increased by a multiple are observed. Atthe same time, corrosion occurs to a certain extent on the ironsurfaces, forming primarily magnetite (rust).

Even with pure magnetite instead of iron filings, the same increasedformation of aldehyde is observed. However, if iron filings or magnetiteare in liquid (for example in the liquid distillation phase), so that nogas-phase contact is possible, then the increased formation of aldehydeis also absent.

The formation of magnetite as a protective coating in plant componentsmade of iron is known per se and is indeed desirable. Under certaincircumstances, however, particles may be detached from a compactmagnetite coating and, having passed to another part of the plant,catalytically promote the oxidation of glycol. Indeed, such magnetiteparticles may occasionally be isolated by filtration in small quantitiesin the course of processes which are in operation. If these particlesarise continuously, then the plant must be protected against corrosionby another means.

By the novel addition of phosphorous acid and/or phosphites to the feedstreams of the distillations it is possible to prevent both corrosion,with formation of magnetite, and the unwanted oxidation of glycol.

The present invention is now illustrated in more detail with referenceto the following working examples.

WORKING EXAMPLES

Using the experiments described in Examples 1 to 4, the effect of theplant material on the formation of aldehyde from ethylene glycol wasinvestigated:

Example 1 Distillation Experiments with Ethylene Glycol

Ethylene glycol (700-800 ml) was first of all distilled at 200 mbar anda liquid-phase temperature of 150-160° C. in a simple distillationapparatus consisting of distillation boiler with boiling capillary,packed column (l=40 cm; d=2.5 cm), descending condenser, receiver anddevice for producing reduced pressure. 13% liquid phase was allowed; theduration of the experiment was about 2 h. Glass rings or iron rings wereused as packing for the column. Either air or nitrogen was bubbled invia the boiling capillary.

The results are compiled in Table 1 below:

                  TABLE 1                                                         ______________________________________                                        Distillation of glycol over glass or iron packing                                            Packing                                                                     Glass          Iron                                                           Aldehyde (ppm).sup.1) 3)                                         Conditions:        free   total.sup.2)                                                                          free total.sup.2)                           ______________________________________                                        Air       Distil-  27     38      42   53                                        late                                                                          Liquid <5 51 30 55                                                            phase                                                                         Balance 24 40 40 53                                                          Nitrogen Distil- 14 25 24 30                                                   late                                                                          Liquid <5 57 12 52                                                            phase                                                                         Balance 13 29 22 33                                                        ______________________________________                                         .sup.1) Initial aldehyde levels: free = 15 ppm; total = 22 ppm                .sup.2) The difference between free aldehyde and total aldehyde is the        socalled bound aldehyde which in the present case, especially in the form     of the acetals, escapes direct determination.                                 .sup.3) The aldehyde levels were determined in accordance with the MBTH       method, a photometric method for free and bound aldehyde (similar to E.       Savicky et al., Analyt. Chem., 33, (1961), 93-96).                       

The overall aldehyde contents which are balanced (ie. obtained takinginto account the liquid phase: distillate ratio of 13:87) from the tableare particularly important for interpreting the analytical data.

In the presence of air, an increase in the total aldehyde content isobserved which, however, is much greater in the case of the ironpacking. Moreover, on this packing a granular, readily movable blackmagnetite coating is formed.

If the distillation is carried out under nitrogen, the formation ofaldehyde is substantially smaller to virtually negligible.

Example 2 Reflux Experiments

In further experiments, glycol was heated under reflux, ie. withoutdistillative removal, in the apparatus according to Example 1 and underotherwise largely identical conditions in this way, contact betweenvapor space and the material being investigated can be maintained over alonger period, so that the posited effects are able to occur to agreater extent.

The column was operated either as a pure glass column (empty) or filledwith iron filings. The latter experimental setup simulates a refluxcondenser of iron or a corresponding evaporator with gas-phase contact.

The results are compiled in Table 2.

                  TABLE 2                                                         ______________________________________                                        Refluxing of glycol in the presence of iron packing in the gas                  phase                                                                           Experiment                                                                              Time               Aldehyde.sup.1) 3)                             No. (h) Atmosphere [ppm]                                                    ______________________________________                                        1         13         Air       162                                              2 14 Air 180                                                                  3 12 Nitrogen  40                                                             .sup.  4.sup.2) 20 Air  38                                                  ______________________________________                                         .sup.1) Total aldehyde                                                        .sup.2) Control experiment in glass                                           .sup.3) Initial aldehyde level 23 ppm                                    

It is observed that, with the ingress of air at the iron surface,ethylene glycol forms a large quantity of aldehyde in a relatively shorttime.

Comparison in particular with the blank experiment (No. 4), whichdespite a prolonged running time (20 hours) shows an increase in thealdehyde content by only 15 ppm, makes clear the connection betweenaldehyde formation and iron surface. In the course of the experiments inthe presence of oxygen, the iron surface becomes covered with a darkoxide layer.

Example 3 Refluxing in the Immersed Phase

Pure monoethylene glycol (MEG) was refluxed in a glass apparatusaccording to Example 1 (p=200 mbar; T=150-160° C.). Iron filings wereplaced in the liquid phase, entirely immersed, and were therefore unableto come into contact with the gas phase. Air was again bubbled in viathe boiling capillary. Table 3 shows the measured total aldehyde levelsat the end of the experiments.

                  TABLE 3                                                         ______________________________________                                        Refluxing of glycol in the presence of iron packings in immersed                phase                                                                                                        Aldehyde.sup.1)                                No. Material Atmosphere [ppm]                                               ______________________________________                                        1      Glass         Air       38                                               2 Glass + iron Air 39                                                          (immersed)                                                                 ______________________________________                                         .sup.1) Initial levels 23 ppm                                            

Accordingly, iron in immersed phase shows the same behavior as the emptyglass apparatus; the increased formation of aldehyde in the previousexperiments must therefore take place in the gas phase.

Example 4 Pretreatment with Phosphorous Acid

In two parallel experiments, a glass column was filled with thefollowing packings:

(1) untreated filings

(2) filings which had been stored overnight beforehand in a 1% strengthsolution of phosphorous acid in glycol.

The experimental procedure was as in Example 2.

The results are compiled in Table 4.

                  TABLE 4                                                         ______________________________________                                        Effect of pretreating the packing with phosphorous acid                                                       Total aldehyde                                  No. Treatment Duration [h] [ppm].sup.1)                                     ______________________________________                                        1      /            13        162                                               2 H.sub.3 PO.sub.3 45  40                                                   ______________________________________                                         .sup.1) Initial levels 23 ppm                                            

With the ingress of air, aldehyde is produced in the case of theuntreated iron filings, with the simultaneous formation and depositionof magnetite.

In the case of the treated iron filings, this corrosion occurs virtuallynot at all and the liquid phase remains almost clear.

The proportion of re-formed aldehyde is substantially lower than in thefirst case.

In the case of treatment with phosphorous acid, the filings becomecoated with a green covering which is retained over the duration of theexperiment.

Example 5 Operational Experiment

An operational experiment was carried out in a continuous productionplant which operates essentially in accordance with the scheme shown inthe attached FIGURE. After H₂ O and ethylene oxide (EO) have beenreacted in the reactor (1) and the crude product has been dewatered inthe drying column (2), phosphorous acid (dissolved in monoethyleneglycol) at a concentration of 20 ppm is metered in continuously to theethylene glycol mixture after it has passed through the evaporator (3)and before it enters the distillation column (4). A marked reduction inthe total aldehyde content in the pure monoethylene glycol (MEG) isobserved (cf. Table 5).

                  TABLE 5                                                         ______________________________________                                        Reduction in the total aldehyde content in monoethylene glycol                  prepared on the industrial scale                                              Aldehyde in the Aldehyde in                                                   Reaction mixture MEG fraction Equilibrium after                             ______________________________________                                        10-30 ppm     <10 ppm    2 hours                                              ______________________________________                                    

At the same time, there is also a reduction in the amount of rust ormagnetite which can be removed by filtration from the liquid-phasedischarge of the distillation.

If desired, H₃ PO₃ can also be metered in directly before thedistillation columns (5) and (6), should the preparation of diethyleneglycol (DEG) or triethylene glycol (TEG) of low aldehyde content berequired.

We claim:
 1. A process for obtaining glycols with low aldehyde contentfrom a glycol-containing mixture, which comprises:treating the surfacesof apparatus component made from corrodible material of a plantassembled for glycol recovery with phosphorous acid or a salt thereof,wherein the phosphorous acid or a salt thereof is added toglycol-containing mixture before or during the working-up of saidmixture; subsequently recovering the glycol(s) by distillation; andoptionally further pre-treating said plant used for the working-up ofsaid mixture, in whole or in part, with said phosphorous acid or a saltthereof, wherein said surface treatment discontinuously takes place atleast in those apparatus components of the plant which, permanently ortemporarily, are in contact with a glycol in vapor form in the course ofwork-up of the glycol product.
 2. The process as claimed in claim 1,wherein the phosphorus compound is metered continuously into theglycol-containing mixture in an amount of about 10 to about 5000 ppm,mixing is conducted, if desired, and then the glycol(s) is(are)separated from the mixture.
 3. The process as claimed in claim 1,wherein the apparatus components of the plant are discontinuouslypretreated, in whole or in part, with a solution which contains thephosphorus compound in an amount of about 0.1 to about 10% by weight. 4.The process as claimed in claim 1, wherein the apparatus components ofthe plant are discontinuously pretreated, in whole or in part, with asolution which contains the phosphorus compound in an amount of about0.1 to about 10% by weight.
 5. The process as claimed in claim 3,wherein the apparatus components of the plant, in whole or in part, isflushed, flooded, irrigated or sprayed with the solution.
 6. The processas claimed in claim 4, wherein the apparatus components of the plant, inwhole or in part, are flushed, flooded, irrigated or sprayed with thesolution.
 7. A process for obtaining glycols with low aldehyde contentfrom a glycol-containing mixture, which comprises:treating the surfacesof apparatus component made from corrodible material of a plantassembled for glycol recovery with phosphorous acid or a salt thereof,wherein the phosphorous acid or a salt thereof is added toglycol-containing mixture before or during the working-up of saidmixture; subsequently the glycol(s) by distillation; and optionallyfurther pre-treating said plant used for the working-up of said mixture,in whole or in part, with said phosphorous acid or a salt thereof. 8.The process as claimed in claim 7, wherein the salt of a phosphorus acidis a water-soluble salt selected from the group consisting of an alkalimetal phosphite, calcium phosphite and zinc phosphite.
 9. The process asclaimed in claim 1, wherein the glycol-containing mixture to beprocessed is a reaction mixture obtained from the production of ethyleneglycol.
 10. The process as claimed in claim 1, wherein theglycol-containing mixture to be processed is selected from the groupconsisting of liquid wastes and residues from chemical production. 11.The process as claimed in claim 10, wherein the liquid waste is selectedfrom the group consisting of used radiator fluid and antifreeze liquids.12. The process as claimed in claim 1, wherein the glycol obtained ismonoethylene glycol, diethylene glycol, triethylene glycol or mixturesthereof, each of a low aldehyde content.